The vast majority, especially of online only games, are just gone for good. And i think open sourcing them, even though it might be quite tricky to build them, is a way better way to go, then just letting it die. And people that are willing to reverse engineer the server part and possibly having lawyers knock on their door, would be willing to figure out how to build something, if that would be another viable option(won't link any PServer of a dead game, because of reasons).
And games like wz2100 wouldn't be where they are today if the code wouldn't have been released. In general i can't think of an example other than 7k2 where no one bothered and tried to build it/get to know the codebase.